The disclosed invention is generally directed to beamsteered phased arrays for radar, and is directed more particularly to a photonically beamsteered phase array that utilizes a photonic true time delay circuit for both transmit and receive functions.
A beamsteered phased array is an antenna system comprising individual antenna elements whose antenna beam direction (i.e., pointing direction) is controlled by the relative phases of the signals respectively radiated or received by the antenna elements. In particular, for transmission, the relative phases of the energy radiated by the individual antenna elements is controlled to define the transmit beam direction of the antenna. For receiving, the relative phases of the energy received by the individual elements is controlled to define the receive beam direction of the antenna. The premise of a true time delay beamsteered phased array is to steer the array beam by introducing known time delays into the respective signals transmitted or received by the respective antenna elements.
Photonic true time delay circuits having optical fiber delay lines have attributes that would make them well suited for phased arrays. Such attributes include relatively wide bandwidth, low loss in the fiber, capability for long delays, small size, low weight, resistance to electromagnetic interference, low cross-talk, and the capability of being remotely located from other components in a phased array.
However, optical fiber delay lines are unidirectional, and thus cannot be used in a manner similar to known bi-directional phase shifters. A relatively direct approach to dealing with such uni-directionality would be to utilize separate optical fiber time delay circuits in the transmit and receive paths of the phased array. However, due to considerations including complexity, noise, and dynamic range, photonic elements might not be acceptable in the receive path which must communicate signals of extremely low power.